1. Field of the Invention
This invention relates to analytical measurement and in particular to planar, glass electrodes for determining cation concentrations in solution. More specifically, this invention relates to multilayer elements for use in the potentiometric determination of ion activity in aqueous liquids, particularly body fluids such as blood sera.
2. Description of Related Art
The related art is replete with a great variety of electrode types and structures for the measurement of various ions in solution. Typically, devices for obtaining such measurement of various ions in solution include a reference electrode and a separate ion-selective electrode. When simultaneously immersed into the same body of solution to be analyzed, the reference and ion-selective electrodes constitute an electrochemical cell, across which a potential develops. This potential is proportional to the logarithm of the activity of the ion of choice which is related to concentration in the solution of the ion of choice to which the ion-selective electrode is sensitive. The foregoing relationship between the potential and ionic activity in solution is described by the well-known Nernst equation. An electrometric device, usually either a direct reading circuit or a null-balance potentiometric circuit, is employed for measuring the potential between the electrodes.
Historically, the ion-sensitive electrode generally comprised an electrode body (usually some type of glass container) containing a known reference solution in contact with a half-cell of known potential, generally Ag/AgCl/"XMCl" and an ion-selective polymeric or glass membrane. The membrane was mounted in an aperture in the electrode body in such a fashion that, when the electrode was immersed in the unknown solution, the membrane contacted both the reference solution within the electrode body and the unknown solution. An appropriate metal probe coated with a layer of an insoluble salt of the metal immersed in the contained reference solution served as the contact while providing a reference potential for the electrode. The selectivity of the electrode was determined by the composition or components of the membrane. Such electrodes are referred to herein as "barrel" electrodes. U.S. Pat. Nos. 3,598,713; 3,502,560; and 3,562,129 provide detailed descriptions of electrodes of this type.
Major shortcomings of some conventional ion-selective electrodes include:
(1) cost: generally a single electrode can be quite expensive; PA1 (2) reproducibility: even with the most carefully performed conditioning procedures, after the first use of the electrode to determine the ionic activity of fluids such as body fluids, the exact composition of the electrode membrane (glass or polymeric) is unknown due to the potential for contamination by earlier test solutions, and for this reason the results are often suspect. PA1 (3) bulk: the barrel electrodes are quite bulky and require a large surface area for operation.
The foregoing problems have been solved to a significant extent by the recent development of dry-operative multilayer ion selective electrodes (ISEs) as described in U.S. Pat. No. 4,214,968 issued July 29, 1980, in the name of C. J. Battaglia et al. These electrodes comprise a dried internal reference electrode in contact with a non-glass hydrophobic ion-selective membrane. The multilayer ISE electrodes described in Battaglia et al can be planar and of uniform thickness in regions of the membrane intended for contact with a sample for analysis. An advantage of the Battaglia dry-operative ion-selective electrode includes its capability of manufacture in a size such as that of a conventional photographic slide transparency. Thus the electrode--or "slide" as it is often referred to--can be low cost and disposable after single use. A glass-membrane containing electrode with at least those advantages would be desirable. The dried internal reference electrode of Battaglia et al is typically composed of one or more dried layers that generally demonstrate minimal, if any, measurable conductivity. In this regard, it will be appreciated that all components in an electrode must be conductive during measurement to provide measurable potential difference representative of chemical activity of species in a liquid under analysis. In the dry operative electrode of Battaglia et al, although the so-called dried layers prior to sample spotting are non-conductive, the sample liquid permeates the membrane and dried layers. Components in the dried layers, for example elctrolyte salts, combine with permeated liquid to provide the necessary conductivity. The internal reference electrode of the present invention, however, must be inherently conductive prior to sample application.
U.S. Pat. Nos. 3,649,506 and 3,718,569, issued Oct. 14, 1969 and Feb. 27, 1973, respectively, describe "solid-state" glass electrodes in which a conductor having a surface layer of an electrochemically active metal is coated with a first coating of a mixture of a molten glass and a halide of the active metal. A second outer coating of molten ion-selective glass is coated over the first coating. The two coatings are thereby bound cohesively to each other. Unfortunately, the high temperatures required to melt and coat the molten glass renders the technique impractical for the preparation of electrodes having components affected by high temperature. Plastic supporting members and other addendum therefore, may be destroyed during the application of molten glass coatings.